battery replacement gone wrong

Most commonly an issue such as yours is related to either the connector on the motherboard or at the end of the screen's flex cable. If you're new to those delicate little plugs, it's pretty easy to damage them by pressing too hard or not getting the plug lined up exactly right before pressing the connector in place.

I'd suggest unplugging the display cable then check it carefully on both sides with a magnifying glass and a bright light. You'll be looking for bent or broken pins; bent ones can sometimes be straightened with fine tip tweezers, but missing pins will either mean replacing the connector on the motherboard or getting a new screen.

Assuming the pins all look straight and present, you'll want to clean both sides of the connector with 90% or higher concentration isopropyl (rubbing) alcohol, give it a minute to dry, then carefully reconnect the display. Pay attention to the comment at this point in the battery replacement guide:

To re-attach press connectors like this one, carefully align and press down on one side until it clicks into place, then repeat on the other side. Do not press down on the middle. If the connector is misaligned, the pins can bend, causing permanent damage.

Give that a try and let us know what happens.